1. Field of the Invention
This invention relates to the preparation of sulfonated polyimide polymers and their use as gas and vapor separation membranes.
2. Description of Prior Art
Polyimides with a rigid chain backbone structure, strong chain-chain interactions, and high glass transition temperatures have been reported to exhibit good gas separation and permeation characteristics in gas separation applications. U.S. Pat. Nos. 4,705,540; 4,717,393; 4,717,394; 5,042,993; and 5,074,891 disclose such aromatic polyimide gas separation membranes. Polyimides are generally soluble in aprotic solvents such as N,N-dimethylacetamide and N-methyl-pyrrolidone. Some polyimides are further soluble in dichloromethane and m-cresol. Their solubility characteristics make polyimides suitable for preparation of asymmetric gas separation membranes but less suitable for preparation of composite gas separation membranes, the latter frequently requiring mild, highly volatile, e.g., low boiling temperature solvents.
A number of modified polyimide polymers have been also disclosed in the art. U.S. Pat. Nos. 4,705,540 and 4,717,394 disclose structurally rigid polyimides having alkyl substituents at all positions ortho to amine functions that form membranes with exceptional gas permeability. U.S. Pat. No. 5,042,993 discloses surface-modified polyimide membranes that exhibit high gas separation factors. The preparation of modified polyimides that contain carboxyl groups is described in U.S. Pat. No. 5,076,816.
The preparation of sulfonated polyimides is reported in U.S. Pat. No. 4,574,144. There was no mention therein of the utility of these materials as separation membranes. The preparation of sulfonated polyimides from sulfonated diamines and pryomellitic dianhydride was further reported by I. V. Vasil'eva et al. in Vysokomol. Soyed., A14, No. 1, 143-149 (1972). The polymers were not soluble in mild solvents such as simple alcohols and were not evaluated for membrane applications. Thus, there remains a need in the field to prepare polyimides with improved separation/permeation characteristics and improved solubility.
It is an object of this invention, therefore, to prepare modified polyimides that have broad solubility characteristics including solubility in mild low-boiling temperature solvent systems, such as alcohols that are most useful in preparation of composite membranes.
It is another object of this invention to prepare modified polyimides with improved gas permeation and separation characteristics.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.